I have heard it argued that emissions from vehicles are insignificant
as factors in environmental damage in comparison with the effects
of volcanic eruptions and that therefore mankind is not responsible
for alleged climate change. How correct is this?

Sara Mikaloff-Fletcher, an atmospheric scientist at NIWA, responded.

Volcanoes emit carbon dioxide, the most important greenhouse
gas produced by human activities. How do these volcanic emissions
compare with emissions from vehicles and other fossil fuel sources?
Fossil fuel emissions are very well known from economic data on how
much fuel is sold and consumed and basic combustion chemistry.
Human beings emitted 36 billion metric tonnes of carbon dioxide in
2013 from fossil fuel burning and cement production. Earth’s volcanoes
have been estimated to produce between 0.13 and 0.44 billion metric
tonnes of carbon dioxide per year. Therefore, volcanic emissions of
carbon dioxide are very small compared to human emissions.

The Earth is a big place, so you might wonder if it is possible that
geologists have missed some volcanic emissions from remote locations.
We can also use measurements of carbon dioxide in the atmosphere
to determine whether it has accumulated more quickly than we would
expect from human made emissions alone and how atmospheric carbon
dioxide levels change after a major volcanic eruption.

Scientists have been measuring carbon dioxide in the atmosphere since 1958.
While there are atmospheric carbon dioxide observing sites all
over the world, the longest continuous record in the Southern Hemisphere
is at Baring Head, New Zealand, which is near Wellington.

These measurements reveal that only about half of all the carbon dioxide
we produce remains in the atmosphere, while the other half is absorbed
by the oceans, plants, and soils. Earth’s natural systems have slowed
climate change, rather than speeding it up.

In the period since atmospheric carbon dioxide measurements began, there
have been three major explosive volcanic eruptions: Agung (1963),
El Chichón (1982), and Pinatubo (1991). After each of these volcanic
eruptions, the rate of carbon dioxide accumulation in the atmosphere slowed.

How could these volcanic eruptions cause carbon dioxide to decrease in the
atmosphere, when volcanoes emit carbon dioxide? Volcanoes emit sulphur
dioxide and other chemicals that cool the atmosphere and change patterns of
cloud and rainfall. These changes affect the way carbon cycles through plants
and soils, leading to additional carbon dioxide uptake by natural systems.

Thank you again for your question. The complex and often unexpected way
that the carbon cycle responds to changes like volcanic eruptions is a big
part of what makes this field of research so fun and exciting to me.